There are numerous devices both implantable and external that have been used to monitor various patient medical conditions. Well known for heart patients is the Holter monitor which permits somewhat uncomfortable monitoring of an electrocardiogram for 24 hours which can then be read by a physician to find anomalies in the rhythm which were not susceptible to discovery or confirmation in a patient's visit to the doctor's office. A number of other devices have improved on the ability to maintain records of electrocardiograms and numerous other health related patient parameters and even device performance parameters. Implantable medical devices such as pacemakers and Implantable Cardioverter/Defribrillators (ICDs) and even non-therapeutic monitoring devices are currently capable of maintaining some records and reporting out such data. An example of a non-therapy delivering monitoring implantable medical device can be seen in U.S. Pat. Nos. 5,313,953 and 5,411,031 issued to Yomtov et al., and in Holsbach et al, U.S. Pat. No. 5,312,446, and others. Nolan et al,'s U.S. Pat. No. 5,404,877 teaches that such devices can even generate patient alarms. All these patents are incorporated herein by this reference in that they provide information about what can currently be done in the implantable device field.
Current generation pacemakers and ICDs have the ability to store different types of information in order to provide feedback to the clinician about the patient/device system. Examples of stored information include arrhythmia diagnostics, histograms of paced and sensed events, electrograms and trends of lead impedance. Such information is useful not only in optimizing device programming but also in the management of the patient's arrhythmias and other conditions. While our invention focuses on the monitoring of patient activity tied to heart rate, which we use as a functional status monitor, the additional information available from implantable devices could be used as an adjunct.
However, to date the literature is devoid of a satisfactory description of how to use activity information. There has been considerable thinking in this area, but none have yet succeeded in producing a satisfactory measure to track patient functional status. Some examples of this thinking in the current literature are described in detail in a application assigned to the assignee of this patent application, with Ser. No. 09/078,221, now U.S. Pat. No. 6,045,513, incorporated herein by this reference in its entirety. This incorporated reference, Ser. No. 09/078,221, now U.S. Pat. No. 6,045,513, also describes in great detail how a simple measure of activity alone can be related to the New York Heart Association classification of CHF patients to provide a status monitor.
As is known in the art, implantable medical devices exist that have various accelerometers and piezo crystal activity sensors and the like which count the movement of the crystal or sensor with respect to a resting state. Medtronic brand implantable medical devices with piezoelectric crystal or accelerometer based activity sensors have the ability to convert a raw activity signal into 2 second activity counts. In other words, the number of times the accelerometer or sensor moves in a two second period is called a 2 second activity count. The prior incorporated '221 application, now U.S. Pat. No. 6,045,513, teaches how to take advantage of such and similar types of data to effectively solve the problems in diagnosis and patient monitoring and tracking a patient's status. It is a well established practice to supply signals from piezocrystals or accelerometers as a measure of metabolic demand.
However, another needed measurement should be available through a device that tracks the patients' natural cardiac responsiveness to stress in a clinically meaningful measurement.
In humans, cardiac output is a function of stroke volume, a reflection of the pumping ability of the heart, and heart rate (sometimes referred to as HR). As people age, or in the presence of conditions that limit the heart's pumping ability, there is a progressive dependence on an increase in HR under conditions such as exercise, during which an increased cardiac output is required. Alternatively, chronotropic incompetence (which we define as an inadequate rise in HR during exercise) may also develop during aging, the progression of heart failure (CHF), or secondary to the use of drugs in the treatment of CHF, hypertension or other diseases. (Such drugs might include beta-blockers, calcium channel blockers, and so forth). This chronotropic incompetence may thus reflect a worsening cardiac status or drug side effects.
Cardiac output is a measurement often useful in characterizing CHF status too. It is measurable by various formulae including the following: EQU CO=(SV)(HR),
where Cardiac Output(CO) is measured in volume (liters) per unit time (minutes), Stroke Volume (SV) is measured in liters per beat and Heart Rate (HR) is measured in beats per minute.
Chronic trend data on HR response to exercise could provide information about the degree of dependence on HR to increase the cardiac output, an indicator of the heart's pumping ability. Over time, an increasing HR response to a given level of exercise may indicate worsening cardiac function. Such exaggerated HR response is a prominent finding in the transition from asymptomatic left ventricular dysfunction to symptomatic heart failure. In other words, if there is a trend toward increasing HR response when a similar level of exercise would not have developed the same high HR rate response, the doctor should be concerned about the onset of symptomatic heart failure and decreasing cardiac output capacity. This trend information may also be clinically useful in tracking the degree of chronotropic incompetence.
While chronic 24-hour mean HR may be also monitored, the change in the mean HR could be reflecting changes in the activity levels of the patient and would therefore not be very useful. Chronic resting HR trends can also provide information about an individual's level of cardiovascular functioning or conditioning, but these data also lack specificity for the problems outlined. As was shown in the Ser. No. 09/078,221 application, now U.S. Pat. No. 6,045,513, monitoring the activity alone may be quite useful clinically. However, changes in patient activity levels may also lack specificity and could be related to orthopedic or other problems or factors.
Currently implantable medical devices have the ability to track HR using intracardiac electrocardiograms or subcutaneous electrocardiograms. Likewise chronic trend information can be collected using a standard piezoelectric crystal or with an implanted accelerometer. While each of these provide some insight, the HR measured and related to the activity level may provide the most specific and clinically useful information about the changing cardiac function. However, currently such a measurement is not provided as a separate measurement.
However, there are no devices that can be implanted and provide long term data with a single variable relating cardiac responsiveness to stress. It would therefore be a valuable adjunct to cardiac medicine to have such a device.
Currently, as patients get sicker from CHF, they typically are less and less active. To test them for CHF, physicians may have them do a standardized exercise routine and look for changes in heart rate response to activity over a series of office visits. Having a device capable of making this evaluation without clinic visits and forcing the patient into a stress test would provide a valuable new tool for cardiac physicians.
It is believed that the CHF patient who is deteriorating and having reduced levels of activity will nevertheless show an increase in a factor we identify as a Heart Rate Activity Coefficient (HRAC), but, if the decrease in activity is due to some other cause the HRAC is unlikely to be affected.
Heart rate variability by itself, as is seen in Schroeppel's U.S. Pat. No. 5,749,900, (incorporated herein by this reference) does not provide the same data on the strength of HR responsiveness to activity or HRAC.